Radio measurements done by a communication device, such as a user equipment (UE), are typically performed on the serving as well as on neighbour cells over some known reference symbols or pilot sequences. The measurements are done on cells on an intra-frequency carrier, inter-frequency carrier(s) as well as on inter-RAT carriers(s) (depending upon the UE capability whether it supports that RAT). To enable inter-frequency and inter-RAT measurements the network configures measurement gaps for the UE, which gaps may comprise time/frequency where the serving cell performs no or limited transmissions (e.g. almost blank subframes).
The measurements are done for various purposes. Some example measurement purposes are to support in: mobility, positioning, self-organizing networks (SON), minimization of drive tests (MDT), operation and maintenance (O&M), network planning and optimization etc. Examples of measurements in 3GPP Long Term Evolution (LTE) are: Cell identification (which is also referred to as PCI acquisition), RSRP, RSRQ, CGI acquisition, RSTD, UE RX-TX time difference measurement, RLM, which comprises Out of synchronization (out of sync) detection and In synchronization (in-sync) detection etc. Channel State Information (CSI) measurements performed by the UE are used for scheduling, link adaptation etc. by network. Examples of CSI measurements or CSI reports are Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), Rank Indicator (RI), etc. They may be performed on reference signals like Cell-specific Reference Signal (CRS), CSI Reference Signal (CSI-RS) or Demodulation Reference Signal (DMRS).
In order to support different functions such as mobility (e.g. cell selection, cell re-selection, handover, etc.), positioning a UE, link adaption, scheduling, load balancing, admission control, interference management, interference mitigation etc., the radio network node, such as an eNode B, also performs radio measurements on signals transmitted and/or received by the radio network node. Examples of such measurements are SNR, SINR, received interference power (RIP), BLER, propagation delay between UE and itself, transmit carrier power, transmit power of specific signals (e.g. TX power of reference signals), positioning measurements like TA, eNode B RX-TX time difference etc.
For example in 3GPP LTE, the DL subframe #0 and subframe #5 carry synchronization signals (i.e. both PSS and SSS). In order to identify an unknown cell (e.g. new neighbour cell) the UE has to acquire the timing of that cell and eventually the physical cell ID (PCI). Subsequently the UE also measures RSRP and/or RSRQ of the newly identified cell in order to use itself and/or report the measurement to the network node. In total there are 504 PCIs in 3GPP LTE.
The UE searches or identifies a cell (i.e. acquires PCI of the cell) by correlating the received PSS/SSS signals in DL subframe #0 and/or in DL subframe #5 with one or more of the pre-defined PSS/SSS sequences e.g. combination of PSS and SSS sequences leading to up to 504 cell identities in LTE. The use of subframe #0 and/or in DL subframe #5 for PCI acquisition depends upon the UE implementation. The UE may further correlate over cell specific reference signals with the pre-defined CRS sequence(s) after the detection of PSS/SSS. The UE regularly attempts to identify neighbour cells on at least the serving carrier frequency(-ies). But it may also search cells on non-serving carrier(s) when configured by the network node. In order to save UE power consumption, the UE typically searches in one of the DL subframes #0 and #5. In order to further save its battery power the UE may search the cell once every 40 ms in a non-DRX or in a short DRX cycle (e.g. up to 40 ms). For longer DRX cycles the UE typically searches a cell once every DRX cycle. During each cell search attempt the UE typically stores a snapshot of 5-6 ms and post-processes this by correlating the stored signals with the known PSS/SSS sequences. In non-DRX the UE is able to identify an intra-frequency cell (including RSRS/RSRQ measurements) within 800 ms (i.e. 20 attempts in total including 15 and 5 samples for cell identification (PCI acquisition) and RSRP/RSRQ measurement).
In device-to-device (D2D) communication, the UEs transmit D2D signals or channels in the uplink part of the spectrum. i.e. either UL resources in TDD or on the UL carrier in FDD. D2D operation by a UE is using a half-duplex mode, i.e. the UE can either transmit D2D signals/channels or receive D2D signals/channels. There may also be D2D UEs, acting as relay nodes, which may relay some signals to other D2D UEs. There is also control information for D2D, some of which are transmitted by D2D UEs and others transmitted by eNodeBs (e.g., D2D resource grants for D2D communication transmitted via cellular DL control channels). The D2D transmissions may occur on resources which are configured by the network or selected autonomously by the D2D UE.
D2D communication implies transmitting, by a D2D transmitter. D2D data and D2D communication control information with scheduling assignments (SAs) to assist D2D receivers of the D2D data. The D2D data transmissions are transmitted according to configured patterns, which may be defined by time and/or frequency, and in principle may be transmitted rather frequently. SAs are transmitted periodically. D2D devices that are within the network coverage may request network node (e.g. eNodeB) resources for their D2D communication transmissions and in response, receive D2D resource grants for SA and D2D data. Furthermore, the network node (e.g. eNodeB) may broadcast D2D resource pools, i.e. the time/frequency assignments in response to the resource requests, for D2D communication.
D2D discovery messages are transmitted in infrequent periodic subframes. Network nodes (e.g. eNodeBs) may broadcast D2D resource pools for D2D discovery, both for reception and transmission. The D2D communication supports two different modes of D2D operation: mode 1 and mode 2. In mode 1, the location of the resources for transmission of the scheduling assignment by the broadcasting wireless device (e.g UE) comes from the network node (e.g. eNodeB). The location of the resource(s) for transmission of the D2D data by the broadcasting UE comes from the eNodeB. In mode 2 a resource pool for scheduling assignment is pre-configured and/or semi-statically allocated. The UE on its own selects the resource for scheduling assignment from the resource pool for scheduling assignment to transmit its scheduling assignment.
PCell interruption of one subframe occurs when the UE switches its reception from D2D to Wide Area Network (WAN) or from WAN to D2D. This is because the UE receiver chain needs to be retuned every time the operation is switched from WAN to D2D reception and from D2D to WAN reception. This applies to both D2D discovery and D2D communication capable UEs. It is important to partition uplink resources between cellular uplink and D2D operation in such a way that avoids or minimize the risk of switching taking place in certain subframes, i.e. subframe #0 and/or #5, of the PCell. These subframes contain essential information, such as PSS/SSS that are necessary for doing cell search, carrying out cell measurements and they also contain Master Information Block/System Information Block (MIB/SIBI) information which is necessary for System Information (SI) reading procedures. In addition to the interruption that takes places due to switching, there may be additional interruption of one subframe due to the RRC reconfiguration procedure. While the switching interruption takes place for UE with a single receiver (RX) (e.g. D2D discovery capable UEs), the RRC reconfiguration interruption takes place for all types of D2D UEs (e.g. D2D Discovery capable and D2D Communication capable UEs). Here, D2D Discovery concerns the ability of devices to discover other devices in their vicinity. The discovery may be network centric or device centric. The purpose of the discovery may be to establish D2D communication or merely to provide information to application layer, for example for a social networking application, that another device is in vicinity. The D2D Communication capability concerns the capability to exchange data directly between devices without going via a network node.
D2D operation is a generic term which may comprise of transmission and/or reception of any type of D2D signals (e.g. physical signals, physical channels etc.) by a D2D communication capable UE and/or by D2D discovery capable UE. D2D operation is therefore also called D2D transmission, D2D reception, D2D communication etc.
A D2D UE is also interchangeably called a Proximity Service (ProSe) capable UE. D2D discovery capable UE is also referred to as UE capable of ProSe direct discovery, and D2D (direct) communication capable UE is also referred to as a UE capable of ProSe direct communication. The radio link and radio carrier that is used for the ProSe direct communication and ProSe direct discovery between UEs is referred to as side link.